To provide a lighting circuit for a vehicular lighting fixture capable of preventing burnout demolition of a control unit and breakage of a light source unit even when a harness connected to a light source unit is short-circuited.

The control unit 20 which controls current flowing in the light source unit 24 is equipped with a current supply part 21 which outputs the current flowing in the light source unit 24, a switching controller 22 in which the current output from the current supply part 21 is switch-controlled as compared with reference voltage, an output current detecting part R1 which detects the current flowing in the light source unit 24, and a voltage conversion circuit 23 in which a potential difference generated by the current flowing in the output current detecting part R1 is converted to a differential voltage to output the converted differential voltage toward the switching controller 22. The output current detecting part R1 is connected to the high potential side of the light source unit 24, and the switching controller 22 compares the reference voltage with the differential voltage and controls the current supply part 21 so that its difference is made smaller.